1. Field of the Invention
The present invention relates to a secondary electron multiplier and more particularly to a secondary electron multiplier having a "Venetian blind" dynode structure for use in a photomultiplier tube or the like.
2. Description of the Prior Art
A Venetian blind dynode structure for multiplying secondary electrons is well known as a dynode arrangement of a secondary electron multiplier in the prior art. the applicant of the present invention disclosed an embodiment of such Venetian blind dynode structure in Japanese Examined Patent Publications No. 23609/1984 and No. 25280/1984.
In the Venetian blind dynode structure, the dynode structure includes a series of slats or vanes disposed at a slanting angle with respect to the direction of propagation of photoelectrons which have been emitted from a photoemissive surface of a photocathode or of secondary electrons which have been emitted from the preceding dynode stage.
FIG. 1 shows an enlarged cross sectional view of a typical example of such Venetian blind dynode structure of a secondary electron multiplier in the prior art. A first dynode 1 includes a pair of thin plates 11 and 12, each capable of emitting secondary electrons. A second dynode 2 includes a pair of thin plates 21 and 22, each also capable of emitting secondary electrons. Thin plate pairs 11 and 12, and 21 and 22, constituting respective first and second dynodes 1 and 2, are slanted at a 45 degree angle with respect to the longitudinal axis of a second electron multiplier tube. Mesh-electrodes 10 and 20 are kept at a same potential as first and second dynodes 1 and 2, respectively.
In FIG. 1, a character "d" refers to the width of a geometrically transparent portion of first dynode 1 in that propagation of the photoelectrons emitted from the photocathode is not hindered by first dynode 1. With "d" is also a free space or gap formed between thin plates 11 and 12, disposed in parallel to each other, constituting first dynode 1. The photoelectrons emitted from the photocathode propagate through this free space to reach second dynode 2.
In FIG. 1, a portion of a thin plate 22 constituting the second dynode 2 having an edge corresponding to an upper edge of thin plate 22 is vertically aligned with the geometrically transparent portions of the first dynode 1. The upper edge of corresponding thin plate 22 refers to one of two edges of thin plate 22, which is disposed relatively close to thin plate 12 constituting first dynode 1.
In FIG. 1, secondary electrons emitted from an upper portion of thin plate 12 constituting first dynode 1 bounce back to thin plate 12, as shown by arrows "a1" and "a2", and secondary electrons emitted from the central portion of thin plate 12 constituting first dynode 1 impinge on the back surface of another thin plate 11 constituting first dynode 1 due to an insufficient electric field, as shown by arrows "b1" and "b2". The upper portion of thin plate 12 refers to a portion of thin plate 12 which is disposed relatively far from thin plate 22 constituting second dynode 2.
Since these secondary electrons do not have a sufficient energy level to be multiplied, these secondary electrons do not contribute to the emission of secondary electrons in the secondary electron multiplier in the prior art. Moreover, the secondary electrons that pass through the geometrically transparent portion of the first dynode and impinge directly on the upper portion of the second dynode also can not be multiplied for the same reason. Therefore, the secondary electron multiplier in the prior art does not provide an efficient electron multiplication.